Frequency modulation altimeter



March 15, 1960 D. J. GREEN FREQUENCY MODULATION ALTIMETER 2 Sheets-Sheet 1.

Filed Jan. 31, 1957 lutz! March' 15, 1960 2 Sheets-Sheet 2 Filed Jan. 31, 1957 'beat or difference frequency signal, fd. 'frequency discriminator combination 18, 20 isY provided 2,929,057 K FREQUENCY MonULA'rroN mammina David J. Green, Beverly Hills, Calif., assigner to Lear, Incorporated Application January 31, 1957, Serial No. 537,535

2 Claims. (Cl. 343-14) This invention relates to distance measuring devices, and more particularlyto frequency modulation altimeters of the closed loop type.

More particularly, this invention relates to closed loop frequency modulation altimeters of the type wherein the rate of frequency change of the transmitted signalis con-V trolled so as to hold the Vbeat frequency of the transmitted and retiected signals substantially constant. Such systems as heretofore known are characterized by complex and costly equipment, and suffer the disadvantage that false readings are provided in the event of failure of certain portions of the system. Y

It is an object of this invention toprovide an improved frequency modulation altimeter of the closed loop type, wherein the likelihood of false altitude indications ris substantially eliminated.

'is pointed out in the appended claims.

In the drawing, Y Y .Y Fig. l is a block diagram of an improved frequency modulation altimeter in accordance with the invention,

Fig. 2 is a block diagram of a modification of a portionv of the altimeter of Fig. 1,

.Fig 3 is a schematic diagram illustrating `a circuit suitable for a portion of the altimeter of Fig. 1,` and v Fig. 4 is a block diagram of anetwork suitable for use in the arrangement of Fig. 2.

' A switching device, indicated as a single-pole doublethrow switch or relay V22,-'is connected between discriml nator VZ0 and an integrator 26, with the relay'arm'atureV 24 connected to integrator 26. Integrator 26 Vpreferably is of the inversesign'type,e.g., a Miller integrator. ,The y 'g Y output of integrator 26 is coupled to a limit sensing del'. Y Y

vice 28 and a Sawtooth generator 30. Limit sensing device ZS' is' adapted to actuate the arin 32 of a further switching device or relay 34 between the terminalsv of a 'D.C. voltage source y36, in a manner hereinafter to be Y i explained.

Briefly, in a frequency modulation altimeter inac. g

cordance with this invention, the beat frequency signal of the transmitted and reflected signals is applied to a frequency discriminator. The output of the discriminator is integrated to provide a Voltage representing the current or instantaneous altitude. This integrated `signal is also utilized to modulate the transmitter, whereby the ,rate of frequency change of the transmittedsignal is controlled to maintain the b eat frequency substantially constant.

Referring to Fig. 1, the altimeterof this invention com- 1 prises a voltage-tunable R.F. source V10 for'transmitting signals from an antenna 12, and receiver means as represented by a mixer-amplier'network 14. The source 1t) is coupled to the mixer-amplier 14, whereby the reiiected signal applied thereto from a receiving antenna i6 is mixed with the locally generated signal to develop the A limiterV and which limits the amplitude of'rtherbeat frequency signal Aand develops a corresponding D .C. voltage,E,V all in a.4 Y

'Conventional m3111161 The armature 24 of,v relay 22 isY adapted, also ina manner hereinafter to be explained, to be actuated by a threshold switch circuitv 38, to which the detector 4 0 of the receiver means is coupled. The arm 32 of switch EhiV is directly connected to one terminal of relay 22, as i1- lustrated.

Source 10 preferably -is a voltage-tunable device, such as a backward-waveroscillator, traveling-wave tube or klystron, having an inherentlyrstable transfer characteristic as represented by the intrinsic relationship between incremental frequency and incremental voltage. A suitable example is the backward-wave oscillator which utilizes a vlong electron drift space between the source cathode and the ultimate anode; located in this space is a periodic Y Y structure capableof propagating microwave energy, and

the electron stream inpassing through this structure asV `it traverses Vbetween the cathode Yand* anode ygenerates 2a microwave signal having a frequency proportional to the dimensions ofthe structureand the electron drift velocity. Since the velocity of the electron! stream is controlled by the voltage impressed between the anode and cathode, the generated frequency'is similarly controlled by this voltage. This represents an-inherent property of Vgenerators of this class, and is substantially independent ofambient temperature and other environmental conditions. If source 10 is modulated by a sawtooth voltageV wave form, a Sawtooth frequency excursion Y i results. For the purpose of this invention, sawtoothvgenerator 30 controls this frequency excursion between predetermined limits; for this purpose, conventional means Vmay be employed for developing a fixed amplitude sawtooth voltage, Vthe slope or repetition rate ofV which changes and corresponds to the input of the generator. In the receiver, the amplifier preferably is sharply Vtuned and has a narrow bandwith centered about theV centerfrequency of discriminator 20. Further, the pass Aband of the amplifier represents only a small fraction of the frequency excursions of the radiated signal.

lt will be apparent that the output voltage, EZ, corresponds to the altitude or distance between thesource 10 and a stationary target from which the radiated wave is reflected. At a given or iixed altitude, the slope ofthe Sawtooth voltage wave form, and hence the rate ofv change of frequency of the radiated wave, is such that the beat frequency is substantially the same asV the center frequency of the discriminator, fo, and the output of the discrimi- 'nator E, is zero.

If the distance tothe target is now decreased, the beat frequency tends to decrease, whereupon a negative D.C. voltage appears in the output of discriminator 20 and effects an' increase in theoutput of integrator 26. Sawtooth generator 30 responds to the increased integrator output to increase the slopeof the Sawtooth voltage wave form, and accordingly to increase the rate of frequency change of the radiated signal between the predetermined limits. This results in an increase in the beat frequency to returnl it to the predetermined value." l To further aid in understanding theoperation ofthe frequency modulation altimeter of this invention, itis noted that since an integrator is used to establish Vthe 1 'voltage indicative of altitude, this measured value can Patented'Mai'.l 15,1960 Y.

grator 26 to D.C. source 36.

be constant only if the output of the discriminator 20 goes to zero immediately following any change in the beat frequency. Therefore, in any fixed altitude situation, it is necessary that the heat frequency be substantially equal to the center frequency of the discriminator. The value of the beat frequency is given by where f is the rate-of-change of the radio frequencyY signal vSince only a narrow range of beatfrequences about fo is of concern in the system of this invention, the advantage of a narrow bandwidth amplifier can be readily appreciated; spurious frequencies resulting from rnechanical vibration of circuit elementsare greatly minimized. Further, the center frequency, fo, of discriminator canrbe chosen at a point where minimum Yspurious microphonic signal interference exists. VAdditionally, if

the center rfrequency fo bisects the pass band of the amplifier, then Vrandom spurious signals will be equally distributed in the positive and negative output regions vof the discriminator and will statistically tend to integrate toward zero effect upon the output signal, EZ.

The arrangement of switches 22, 34, limit-sensing and threshold switches 2S, 38, D.C. source 36 and detector 40 comprise means for initially determining altitude when the altimeter system is activated. Threshold switch '38 controls the switch 22 in accordance with the magnitude of the output of detector 40, and for this purpose may comprise Vsimple relay means operable in the deenergized condition to connect integrator 26 to discriminator 2i), and in the energized condition to connect inte- Preferably, threshold lswitch 38 connects switch arm 24 to D.C. source 36 so ylong as the output of detector 40 is below a predetermined level, and to the output of the frequency discriminator 20 for detector outputs above that level.

Limit sensing switch 28 responds to the output of integrator 26 to move switchrarm 32 between the terminals of D.C. source 36. [For this purpose, limit sensing switch 28 preferably comprises diode and relay switching means operable at different voltages to actuate switch arm 32; at a predetermined minimum integrator output corresponding to minimum or zero altitude, switch arm 32 is connected to the negative terminal of source 26, and at a predetermined maximum integrator output corresponding to the maximum permissible altitude reading for the system, switch arm 32 is connected to the positive terminal of source 26. Y

One arrangement for limit sensing switch 28V is shown in Fig. 3. Referring to Fig. 3, a unidirectionally conductive device, shown as a diode 60, is connected between integrator 26 and one end of a relay control coil 62. The other end of coil 62 is connected to the switch arm 64 of a single-pole double-throw switch y66, for

vrswitching between two points of D.C. potential V1 and V2. Coil 62 is adapted to actuate switch arm 64 when 1 energized; in the de-energized condition of coil 62,.switch arm 64 is normally in contact with the point of higher potential, V1. Further, switch arm 32 is in contact with Y r'the negative terminal of D.C. source 36 Vin the de- V eenergized condition of coil 62. As illustrated, the anode 68-1 of diode 60 is connected to integrator 26, and the cathode 70 is connected to the one end of coil 62.

While the integrator output voltage,` Ez, is negative Y with respect to potential V1, the diode is non-conductive. When the output voltage EZ reaches the potential V1,

diode 60 conducts. Current liow throughl coil V62 causesV l' the switch arm 64 to be connected to the point of lower D.C. potential, V2, and the switch arm 32 to be moved to the positive terminal ofD.-C. sourceV 36. When this occurs, the integrator output decreases until it reaches the potential V2, whereupon diode 6) ceases conduction, switch arm 64 is connectedrto potential V1, and switch arm 32 is returned to the positive .terminal of D.C. source 36. Potentials V1. and V2 are chosen topermitthe Y to range through all desired 4i output of integrator 26 altitude values. Y

It will be apparent that the circuit of Fig. 3 represents merely one arrangement to effect switchingof integrator 26,between points of dierent potential, and that any switching means for this purpose is embraced by this f invention. Y

y The action of integrator 26 in integrating the AD.C. i Y voltage is to cause the output voltage Ezrto sweep Yat av of EZ to sweep in the opposite direction. In this manner, the output voltage Ez is caused to sweep, or scan, in a recurrent fashion through all altitude values.Y

While thus scanningrthrough all the possible altitude` values, detector 40 monitors the beat frequency signal to detect the presence of a signal to be applied to the integrator, i.e., a beat frequency signal fd substantially equal to the discriminator center frequency fo.. When the desired beat frequency signal is'approached, the output voltage'of detector 4G reaches'a level suicient to cause threshold switch 38 to connect switch arm24 to the output of discriminator 20. Thereafter, the system operates as previously described. If loss of signal occurs, threshold switch 38 automatically connects switch arm 24 to D.C. source 36, whereupon scanning takes place to eifect lock-on when signals are again present.

' Fig. 2 villustrates an alternative operation to V'elfect initial lock-on of the system.V Referring to Fig. 2, thev output of discriminator 20 and Ythe D.C. source (through switch 34) are connected to an equality sens-` ing switch S0 which continuously compares the '.magni Vtude ofY the discriminator output with the D.C. po-

tential. This switch holds switch arm 24 in contact with the D.C. source until the discriminator output reaches the potential of the terminal of the D.C. source to which switch arm 32 is connected; at this point, switch arm 24 is connected to the output of discriminatorz. This lock-on control circuit'does not have. the feature of automatically connecting switch arm 24Vto ,the D.C.

source upon loss of signal; however, Va detector may be employed in conjunction with this circuit to monitor 'the beat frequency signal and thereby provide automaticf scanning under such conditions, as in the manner shown in Fig. 1. Y Y

Fig. 4 illustrates one network for the equality sensing switch 50. As shown, this arrangement comprises Aa Ybalanced modulator 74, a detector76 anda relay 78,

It will be apparent from the foregoing that afsimple but if Y highly ehcient frequency modulation altimeter has been f described. False distance indications are highly unlikely,

Y inasmuch as 'failure .or inoperativeness of circuitry within the loop will eitherresult in failure to develgp 4 All,

output signal or in automatic scanning to continuously vary the output signal and thereby indicate that output voltage readings are not representative of true distance measurement.

What is claimed is:

` 1. A distance determining system utilizing reected radio waves comprising a radio transmitter for transmitting a radio wave, means to develop a sawtooth voltage for frequency modulating said transmitter, means for receiving said wave after reection from an object, means to combine the transmitted and reflected'waves to produce a beat frequency signal, a frequency discriminator having a center frequency of a predetermined constant value through which the beat frequency signal is passed, means for integrating the output of said discriminator to obtain a control signal, means to apply said control signal to said frequency modulating means for controlling the frequency of the sawtooth voltage so as to hold said beat frequency signal substantially to said predeterminedV constant value, a D.C. source having positive and negative terminals, a first switch, limit sensing means connected to said integrating means and operable at predetermined control signals to actuate and move said switch from one to the other of said terminals, a detector, means for applying said beat frequency signal to said detector, a second switch connected to said integrating means and adapted in respective irst and second positions to connect said integrating means to said D.C. source and to said discriminator, means coupled to said detector and operable to place said second switch in said first position when the beat frequency signal is substantially different from said predetermined constant value and in said second position when the beat frequency signal is substantially at said predetermined constant value.

2. A distance determining system utilizing reflected radio Waves comprising a radio transmitter for transmitting a radio wave, means to develop a sawtooth voltage for frequency modulating said transmitter, means for receiving said Wave after reflection from an object, means to combine the transmitted and reflected waves to produce a beat frequency signal, a frequency discriminator having a center frequency of a predetermined constant value through which the beat frequency signal is passed, means for integrating the output of said discriminator to obtain a control signal, means to apply said control signal to said frequency modulating means for controlling the frequency of the sawtooth voltage so as to hold said beat frequency signal substantially to said predetermined constant value, a D.C. source having positive and negative terminals, a rst switch, means connected to said integrating means and operable at respective predetermined control signals corresponding to maximum and minimum distances to be measured to actuate and move said switch from one to the other of said terminals, a second switch connected to said integrating means and adapted in respective first and second positions to connect said integrating means to said D.C. source and to said discriminator, means to compare voltages from said discriminator and said D.C. source when the system is initially activated, said voltage comparing means holding said second switch in said first position initially, and said voltage comparing means moving said switch to said second position when the voltages from said discriminator and said D.C. source are substantially equal.

References Cited in the file of this patent UNITED STATES PATENTS 2,537,593 Landon et al. Jan. 9, 1951 2,726,383 Dunn Dec. 6, 1955 2,748,272 Schrock May 29, 1956 

